What Exactly Are Resonance Structures in Chemistry?

[loom91]

Hi,

I'm confused by resonance structures of molecules, what are they really? My teacher said resonance structures are in fact a decomposition of the actual molecular wave-function as a sum of simpler ones. Is this true? Or are they in fact simply pictures we draw when we can not classify the molecule into our simplistic lewis structures as claimed by my textbook? Thanks!

Molu

 

[Gokul43201]

Strictly speaking, they are essentially what your teacher has described - trial wavefunctions written as a linear superposition of simpler (or better known) functions, with the ratios of coefficients variationally optimized to yield the state with lowest energy. If the measured bond energy of the molecule is pretty close to this calculated minimum value, you can claim to have a good approximation to "real" wavefunction.

Pauling explains this well in the early chapters of (The Nature of) The Chemical Bond.

However, it is true that a precursor to the more rigorous concept of resonance was in use for a few decades before the necessary quantum mechanics was developed. For the most part, one can intuitively (mostly out of practice) guess what the "resonating" structures are. But there is no real way, short of doing the very difficult calculations, of guessing the contributions of the different structures (ie, the ratios of the coefficients).

 

[loom91]

Thanks, that cleared it up. I guess one tackles the actual resonance calculations in VB? Are they tackled at UG or PG level?

 

[Gokul43201]

Thanks, that cleared it up. I guess one tackles the actual resonance calculations in VB?

What's "VB"?

 

[loom91]

VB = Valence Bond Theory
MO = Molecular Orbital Theory

 

[Gokul43201]

Thanks, that cleared it up.

I actually wasn't attempting to clear it up entirely - what I gave you was hardly a semi-synopsis. If you do have access to a library, find Pauling's book. You won't regret it.

I guess one tackles the actual resonance calculations in VB? Are they tackled at UG or PG level?

I'm not sure exactly when and if these concepts get "taught in class." If you go through a conventional Physics education, you might have all the tools necessary to do the calculation for simple resonances (the singly ionized Hydrogen molecule - H₂⁺ - for starters) by your second or third semester of QM.